1. Field of the Invention
The present invention relates generally to amphiphilic biodegradable block copolymers and self-assembled polymer aggregates formed from the same in aqueous milieu. More particularly, the present invention relates to amphiphilic biodegradable block copolymers comprising polyethylenimine (PEI) as a hydrophilic block and aliphatic polyesters as a hydrophobic block, and self-assembled polymer aggregates formed from the block copolymers in aqueous milieu.
2. Description of the Related Arts
Recently, nanostrucured materials have received much attention as a potentially effective drug carrier. Therefore, various amphiphilic polymers comprising both a hydrophobic block and a hydrophilic block have been synthesized in order to develop effective nanostructures. In aqueous milieu, the hydrophobic compartment of an amphiphilic polymer has a tendency to self-assemble in order to avoid water and to minimize free interfacial energy of the system. By this hydrophobic interaction, the amphiphilic polymers form self-assembled aggregates in aqueous milieu. In addition, the hydrophilic blocks are uniformly dissolved in aqueous milieu and thereby the aggregates maintain a thermodynamically stable structure.
As compared with conventional low-molecular-weight micelles, polymer aggregates can form a more stable structure by chain entanglement and crystallinity of polymers, which is the reason why synthetic polymers have been extensively used as a material for drug delivery vehicles. Particularly, because the formed structure is uniform and nano-scale, it can be applied as a targetable drug delivery system, a carrier micelle for solubilizing insoluble drugs, and a gene delivery system.
These amphiphilic polymers exhibit various properties depending on their component blocks. For example, physicochemical properties of amphiphilic polymers and structures formed therefrom are determined by a molecular weight of the polymer, a ratio of hydrophilic/hydrophobic blocks, rigidity of blocks, affinity between blocks, molecular structure of blocks, charge density of a hydrophilic block, addition of ligands, etc.
In connection with a drug delivery system, many studies have been conducted on hydrophilic polymers. Hydrophilic polymers such as poly(ethylene oxide) (PEO) or poly(ethylene glycol) (PEG) have an excellent biocompatibility. In particular, there are many reports regarding the addition of poly(ethylene oxide) into various hydrophobic blocks.
For example, R. Langer of MIT synthesized polymer nanoparticles comprising polyethylene oxide as a hydrophilic block and biodegradable polyester as a hydrophobic block. The polyesters such as polylactide or poly(D,L-lactide-co-glycolide) was approved as a biodegradable and biocompatible polymer for clinical uses by Food and Drug Administration (FDA) in USA. In this case, we can observe that poly(ethylene oxide) drifts to the surface of nanoparticles by phase separation and drug concentration effused in blood increases in comparison with that case when polymer particles without poly(ethylene oxide) was used[R. Gref, Y. Minamitake, M. T. Peracchia, V. Trubeskoy, A. Milshteyn, J. Sinkule, V. Torchilin, R. Langer, Int. Symp. Controlled Release Mater., 20, 131 (1993)].
Additionally, there have been reported the preparation of self-assembled polymer aggregates using poly(ethylene oxide) as a hydrophilic block, and as for a hydrophobic block, for example, using poly(xcex2-benzyl-L-aspartate) [G. S. Kwon, M. Naito, M. Yokoyama, T. Okano, Y.Sakurai, K. Kataoka, Langmuir, 9, 945 (1993)]; using poly(propylene oxide) [A. V. Kabanov, E. V. Batrakova, N. S. Neiknubanov, et al. Journal of Controlled Release, 22, 141 (1992)]; using poly(xcex5-caprolactone) [C. Allen, J. Han, Y. Yu, D. Maysinger, A. Eisenberg, Journal of Controlled Release, 63, 275 (2000)]; using oligo-methacrylate [T. Inoue, G. Chen, K. Nakamae, A. S. Hoffman, Journal of Controlled Release, 51, 221 (1998)]; etc.
Recently, there have been reported the preparation of polymer aggregates using ionic interactions. K. Kataoka et al. proposed a novel concept of polymer aggregates, xe2x80x9cpolyion complex (PIC) micellesxe2x80x9d formed by ionic bonding between polymers having counter ions, by using both poly(ethylene oxide)-poly(L-lysine) block copolymer and poly(ethylene oxide)-poly(L-aspartate) block copolymer [A. Harada and K. Kataoka, Macromolecules, 28, 5294 (1995)]. By using this concept, they reported that lysozyme, which is a protein having an isoelectric point of 11 and thereby has positive charge, is successfully loaded within polymer micelles [A. Harada and K. Kataoka, Macromolecules, 31, 288 (1998)].
In addition to core-shell type polymer micelles using poly(ethylene oxide), there have been many concerns on structures such as cylindrical micelles, hollow vesicles, hollow hoops, etc. Adi Eisenberg (McGill University in Canada) reported various types of polymer aggregates using poly(ethylene oxide)-polystyrene copolymer [K. Yu and A. Eisenberg, Macromolecules, 29, 6359 (1996)]. In addition, D. E. Discher and D. Hammer (Pennsylvania University in USA) proposed a novel vesicular structure, xe2x80x9cpolymersomexe2x80x9d using poly(ethylene oxide)-poly(ethylethylene) block copolymer [B M Discher, Y Y Won, D. S. Ege, J. C-M. Lee, F. S. Bates, D. E. Discher, D. A. Hammer, Science, 284, 113 (1999)].
All the above polymer aggregates use poly(ethylene oxide) as a corona block in consideration that poly(ethylene oxide) is a nonionic polymer without reactivity with in vivo biological molecules and particularly, poly(ethylene oxide) having molecular weight of 5,000 or less can be filtrated at kidney to be discharged to the exterior. Thanks to such an excellent biocompatibility of poly(ethylene oxide), polymer aggregates using poly(ethylene oxide) have been extensively considered as a polymer material for drug delivery system. In particular, because poly(ethylene oxide) inhibits protein adsorption, polymer aggregation can be prevented from interaction with in vivo molecules in blood, thereby can be protected from removal by immunocytes such as mononuclear phagocyte system (MPS) and can stay in blood for a long time.
However, poly(ethylene oxide) does not have other functional groups except terminal groups, so to be limited to attach cell-adhesion molecules in application of a targetable drug delivery system. In addition, in application of oral or percutaneous administration, it is difficult to increase penetration into the tissue due to a large hydrodynamic volume of poly(ethylene oxide). Besides, there is a limitation in forming various structures of polymer aggregates. In addition, there is defect that block length has to be longer for forming polymer aggregates in comparison with ionic polymers, so that the volume of core part to contain drug is relatively small. Therefore, there have been needs for novel polymer aggregates using another polymers different from poly(ethylene oxide), depending on drug administration routes.
Aggregates formed from polymer electrolytes have been applied as a gene delivery carrier. However, electric charge of the polymer is used in coupling between a polymer and a gene, and therefore, it does not determine surface property of the aggregates.
Adi Eisenberg et al. (McGill University, Canada) reported various structures having a charged hydrophilic polymer as a corona block, wherein the charged polymer exists on the surface of the self-assembled aggregates. He employed polystyrene as a hydrophobic block and poly(acrylic acid) as a hydrophilic block. In this case, polymer aggregates can be formed from poly(acrylic acid) having much lower molecular weight compared with the conventional corona block such as poly(ethylene oxide). This structure is named as xe2x80x9ccrew-cutxe2x80x9d polymer aggregates [L. Zhang and A. Eisenberg, Science, 268, 1728 (1995); L. Zhang, K. Yu, A. Eisenberg, Science, 272, 1777 (1996); L. Zhang and A. Eisenberg, Macromolecules, 29, 8805 (1996)]. In this case, the length of hydrophobic blocks can be controlled to be relatively longer and various types of structures can be formed depending on block length or molecular weight of polymer, condition of aqueous milieu, etc. [C. Allen, D. Maysinger, A. Eisenberg, Colloids and Surfaces B: Biointerfaces, 16, 3 (1999)]. However, polystyrene is not a biocompatible polymer and has difficulty in removal after injection into the body.
Therefore, a biocompatible polymer is required in order to be applied as a drug delivery system. Biocompatible polymers must not cause in vivo inflammation or immune reaction, must be biodegraded in vivo to be easily removed, and degradation products thereof must be harmless in vivo. As a polymer satisfying these conditions, biodegradable aliphatic polyesters using lactic acid or glycolic acid as monomer units were approved by FDA. Aliphatic polyester has been extensively used as a drug delivery carrier or a surgical suture, and has been verified biocompatible.
Under this circumstances, in order to solve the above-mentioned problems, the present inventors have conducted extensive studies on novel polymer aggregates using other polymer different from poly(ethylene oxide) as a hydrophilic block. As a result thereof, they found that polymer aggregates comprising cationic polyethylenimine as a hydrophilic block and an aliphatic polyester with biodegradability and biocompatibility as a hydrophobic block can satisfy the above object.
Therefore, one object of the present invention is to provide biodegradable block copolymers comprising polyethylenimine as a hydrophilic block and aliphatic polyesters as a hydrophobic block and to provide polymer aggregates formed therefrom.
Another object of the present invention is to provide multifunctional block copolymers to be applied in vivo utilizing electric charges and functional group of hydrophilic block.
These and other objects and advantages of the present invention will become apparent to the skilled in the art from the following detailed description as considered in conjunction with the accompanying drawings.